DocID024112 Rev 1

7/26

AN4229

Vocoder overview and comparison

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The features of Speex include:

•

Three different sampling rates: narrowband (8 kHz), wideband (16 kHz) and ultra-

wideband (32 kHz)

•

Wide range of bitrates available (from 2 Kbit/s to 44 Kbit/s)

•

Quality: ranges from 0 to 10

•

Variable complexity: ranges from 1 to 10

•

Variable bitrate (VBR)

•

Voice activity detection (VAD)

•

Perceptual enhancement…

Speex is targeted at voice over IP (VoIP) and file-based compression. The design goals

have been to make a codec that would be optimized for high quality speech and low bit rate.

Also, several other applications support the Speex codec such as streaming applications,

videogames and audio processing applications.

1.5

Vocoder comparison

Table 2 summarizes and compares general and technical information for the previously

presented codecs.

Table 2. Codecs comparison

Codec name

G.711

G.726

IMA-ADPCM

Speex

Creator

ITU-T

ITU-T

IMA (interactive

multimedia

association)

Xiph.Org foundation

Algorithm

Companding A-law or

μ-law, PCM, Lossy

ADPCM, Lossy

Adaptive differential

pulse code

modulation)

CELP (code excited

linear prediction)

License

GPL

GPL

Free

Patent-free (BSD

license)

Bitrate (Kbits/s)

64

16, 24, 32, 40

32

2.15 to 24.6 (NB)

4 to 44.2(WB)

Sampling rate (kHz) 8

8

8

8, 16, 32

Bits per input sample 16

13

16

16

Bits per output

sample

8

2, 3, 4, 5

4

Latency: 30 ms (NB),

34 ms (WB)

Application

– Voice recording

and playback

–VoIP

– Automated

announcement

systems

– Intercom

– Walkie-talkie

– Voice recording

and playback

–VoIP

– Automated

announcement

systems

– Speech

compression in

telecommunications

– Voice recording

and playback

–VoIP

– Streaming

applications

(teleconference)